Here's why:
* "Whom" is the objective case of the pronoun "who". It's used when the pronoun is the object of a verb or preposition.
* "Who" is the subjective case, used when the pronoun is the subject of a verb.
In the phrase "running from whom," "whom" is the object of the preposition "from".
Example:
* "I don't know whom she's running from."